1 QEMU 9p virtfs proxy filesystem helper
2 ======================================
7 **virtfs-proxy-helper** [*OPTIONS*]
12 NOTE: The 9p 'proxy' backend is deprecated (since QEMU 8.1) and will be
13 removed, along with this daemon, in a future version of QEMU!
15 Pass-through security model in QEMU 9p server needs root privilege to do
16 few file operations (like chown, chmod to any mode/uid:gid). There are two
17 issues in pass-through security model:
19 - TOCTTOU vulnerability: Following symbolic links in the server could
20 provide access to files beyond 9p export path.
22 - Running QEMU with root privilege could be a security issue.
24 To overcome above issues, following approach is used: A new filesystem
25 type 'proxy' is introduced. Proxy FS uses chroot + socket combination
26 for securing the vulnerability known with following symbolic links.
27 Intention of adding a new filesystem type is to allow qemu to run
28 in non-root mode, but doing privileged operations using socket IO.
30 Proxy helper (a stand alone binary part of qemu) is invoked with
31 root privileges. Proxy helper chroots into 9p export path and creates
32 a socket pair or a named socket based on the command line parameter.
33 QEMU and proxy helper communicate using this socket. QEMU proxy fs
34 driver sends filesystem request to proxy helper and receives the
37 The proxy helper is designed so that it can drop root privileges except
38 for the capabilities needed for doing filesystem operations.
43 The following options are supported:
45 .. program:: virtfs-proxy-helper
51 .. option:: -p, --path PATH
53 Path to export for proxy filesystem driver
55 .. option:: -f, --fd SOCKET_ID
57 Use given file descriptor as socket descriptor for communicating with
58 qemu proxy fs drier. Usually a helper like libvirt will create
59 socketpair and pass one of the fds as parameter to this option.
61 .. option:: -s, --socket SOCKET_FILE
63 Creates named socket file for communicating with qemu proxy fs driver
65 .. option:: -u, --uid UID
67 uid to give access to named socket file; used in combination with -g.
69 .. option:: -g, --gid GID
71 gid to give access to named socket file; used in combination with -u.
73 .. option:: -n, --nodaemon
75 Run as a normal program. By default program will run in daemon mode